Rhesus monkey hosts were used in intraperitoneal host-mediated assay (HMA) of mitomycin C with E. coli K-12, 343/113 cells enclosed in dialyzing tube diffusion bags. Intrasanguineous HMA or blood-mediated assay failed due to low survival of indicator cells. The animals used were also evaluated cytogenetically for chromosomal aberrations in somatic and germinal tissues. MC mutagenicity was detected in each.